The invention relates to a method of synchronizing the drive applied to a gas-insulated circuit breaker with the waveform of the voltage across the terminals of the circuit breaker so as to cause the circuit breaker to switch at a target instant that is as close as possible to a predetermined instant corresponding to a certain amplitude level of the voltage waveform.
By way of example, such synchronization makes it possible to close the circuit breaker at the instant when the level of the voltage waveform across the terminals of the circuit breaker is close to zero.
Until now, the drive to a gas-insulated circuit breaker has been synchronized by applying time compensation to circuit breaker drive as a function of ambient temperature, feed voltage to circuit breaker control auxiliary equipment, etc., as measured immediately before driving a switching operation of the circuit breaker. All of those measured parameters have an effect on the duration of the time required to drive the moving contact(s) of the circuit breaker and need to be taken into consideration in order to adjust the instant at which the drive ought to be applied so as to obtain circuit breaker switching as close as possible to the predetermined instant. Naturally, synchronization requires behavior of the voltage waveform to be monitored continuously so that circuit breaker drive can be initiated at the appropriate moment given the compensated drive time as computed and the target switching instant.
FIG. 1 is a graph showing how the alternating voltage waveform TR across the terminals of the circuit breaker varies for one phase of the circuit breaker. On this graph to indicates the instant at which a switching order is sent to the circuit breaker control, t1, indicates the instant at which circuit breaker drive is engaged by the control, and tc indicates the instant at which the circuit breaker switches. In this figure, instant tc corresponds to an instant when the voltage is zero. The instants t1 and tc are separated by a time interval te which corresponds to the compensated drive time tmc as computed by the synchronization device on the basis of measurements of ambient temperature, feed voltage to control auxiliary units, etc. . . . . The instants t0 and t1 are separated by a time interval td corresponding to a drive-engagement time delay running from the switch order so as to ensure that switching is synchronized with a voltage zero.
FIG. 2 is another graph in which curve TR shows how the absolute value of the alternating voltage across the terminals of the circuit breaker varies over time. This graph also shows a curve C1 representing variation in the dielectric characteristic of the circuit breaker during a closure operation when the density of the insulating gas in the circuit breaker is at its lowest critical value, and curve C2 shows how the dielectric characteristic of the circuit breaker varies during a closure stage when the density of the insulating gas in the circuit breaker is at a nominal value above the critical value. The curves C1 and C2 are the two characteristic curves of circuit breaker dielectric characteristic (or of electric arc pre-striking between the two contacts of the circuit breaker) and they demonstrate that the dielectric characteristic of the circuit breaker decreases as the contacts of the circuit breaker move towards each other, until the circuit breaker has closed completely. In practice, in order to synchronize circuit breaker drive, the target switching instant tc is computed by taking account of the curve C1, and as a result this target instant is offset from voltage zero and circuit breaker switching takes place at an instant when the voltage across the terminals of the circuit breaker is not zero. In the example represented by curves C1 and C2 in FIG. 2, it can be seen that switching takes place at an instant when the voltage lies in the range 0.16 to 0.19 of the nominal voltage, and in practice is close to 0.19 of the nominal voltage.
The object of the invention is to propose an improved synchronization method which makes it possible to obtain switching closer to voltage zero (or any other selected level in the voltage waveform). More particularly, the object of the invention is to optimize computation of the target instant.
To this end, the invention provides a method of synchronizing drive to a gas-insulated circuit breaker with the voltage waveform applied to the terminals of the circuit breaker so as to cause the circuit breaker to switch at a computed target instant that is as close as possible to an instant corresponding to a certain amplitude level in the voltage waveform, wherein the pressure of the insulating gas inside the circuit breaker is measured immediately before said switching, and wherein said measurement is used together with prerecorded data representative of variation in the dielectric characteristic of the circuit breaker as a function of the pressure of said insulating gas in order to optimize computation of said target instant.
As explained above, the dielectric characteristic of the circuit breaker varies as a function of the pressure of the insulating gas between two extreme values, C1 corresponding to the critical pressure value (minimum pressure), and C2 corresponding to the nominal pressure value. Between these two extreme values, the dielectric characteristic C of the circuit breaker varies as a function of the pressure of the insulating gas. FIG. 2 shows that in the prior art, the optimum target time used to be calculated as a function of the value C1. However, if the insulating gas pressure inside the circuit breaker is measured immediately before applying drive to cause the circuit breaker to switch, it is possible to compute a target instant that is closer to voltage zero than the target instant given by curve C1. In general, the way the dielectric characteristic of a circuit breaker varies as a function of variation in the pressure of the insulating gas inside the circuit breaker can be represented approximately by a polynomial or other function and this function can be recorded in the form of data for defining the curve C that represents the dielectric characteristic of the circuit breaker for any given pressure of the insulating gas. On the basis of the curve C, it is likewise possible to compute the corresponding target instant. Thus, the accuracy of synchronization can be improved.
In a particular implementation of the method of the invention in which the gas-insulated circuit breaker is driven by a hydraulic control, the hydraulic pressure is measured immediately before switching the circuit breaker, and said hydraulic pressure measurement is used together with the prerecorded data representative of variation in the dielectric characteristic of the circuit breaker as a function of hydraulic liquid pressure so as to optimize computation of said target instant. Variation in the dielectric characteristic of a circuit breaker as a function of variation in the pressure of the hydraulic liquid is represented in a manner analogous to that used for variation in the pressure of the insulating gas, except that it is also proportional to the displacement speed of the contacts which itself depends on the pressure of the hydraulic control liquid.